internal/singleflight: format someErr

Error string should not be capitalized.

Change-Id: I8e1d148c6b999450bcd702f420c2a240f82aadc7
GitHub-Last-Rev: 6ca1b3edb4a61723fa6472a0f54cc6329898edbc
GitHub-Pull-Request: golang/go#35147
Reviewed-on: https://go-review.googlesource.com/c/go/+/203339Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/singleflight/singleflight_test.go

@@ -28,7 +28,7 @@ func TestDo(t *testing.T) {
 
 func TestDoErr(t *testing.T) {
 	var g Group
-	someErr := errors.New("Some error")
+	someErr := errors.New("some error")
 	v, err, _ := g.Do("key", func() (interface{}, error) {
 		return nil, someErr
 	})